There’s more than meets the eye when it comes to painting fresh stucco. Stucco is unique in that it requires chemical curing for proper paint-coating adhesion. Humidity, rain, sun exposure and wind can affect the entire process. Here are a few things to keep in mind when taking on a job where fresh stucco is being applied.

KNOW YOUR PH

Understanding the pH level of stucco cement is critical. A balanced pH is 7; any number below that is acidic, and a higher number is alkaline. Fresh stucco may have a pH of 12 or higher when first applied, but you can easily monitor progress with a kit from your local paint distributor. To do a field test, moisten the surface with distilled water (not tap water), and stroke the surface with a pH pencil. The resulting color is matched to a chart to assess the pH level.

“Even though the paint label says to wait 28 days before going over plaster or cement, you want to get as close to a neutral pH as possible,” says Larry Baker, owner of Think Stucco in Minneapolis/St. Paul.

Beyond time recommendations, heed the coating manufacturer’s label for how low the pH needs to be. So-called ‘hot primers,’ which are extremely alkali resistant, can be applied at higher pH levels.

DON’T PAINT TOO SOON

Applying paint early can create several problems at the chemical level:

• Lime or alkali burn. This causes color loss, bleaching, chalkiness, or other deterioration of the coating pigments.
• Efflorescence. Moisture inside a wall can carry salts to the surface, leaving behind white deposits or crystals. With a highly permeable coating, the salts may bleed through. Less permeable coatings can trap the deposits and cause delamination, or separation between the coating and surface.
• Saponification. High alkalinity and moisture can also attack the coating resin chemically causing blisters, brittleness, or a soapy texture. (In fact, ‘sapo’ is the Latin word for soap.)

A combination of chemical, moisture and adhesion tests can all play a role in determining when it’s safe to put down a coating—and help a paint pro avoid the problems that can occur from a rushed time line.